(A) Field of the Invention
This invention relates to polyurethane coating compositions and more particularly relates to high performance one-component polyurethane compositions with excellent weathering properties.
(B) Description of the Related Art
It has been known that polyurethanes can be formed by the reaction of isocyanates with active hydrogen compounds such as polyols (polyfunctional alcohols). Since isocyanates and active hydrogen compounds usually react very quickly even at room temperature, the two components to form the polyurethane compositions usually must be stored separately and mixed together only at the time when the coating is to be used.
Several approaches have been investigated to provide a one-component composition in which the reacting materials are combined before using and then later activated. For example, prepolymer isocyanates have been converted into derivatives of isocyanates which regenerate the isocyanate only upon heating. This heating process has several disadvantages especially when the polyurethane is to be used as a coating. It is, for example, very difficult to heat a very large surface or to heat a surface which is temperature sensitive.
Another technique for the manufacture of one-component polyurethane systems is the formation of moisture cure systems. This process involves the formation of an isocyanate prepolymer which reacts with moisture from the atmosphere to form a polymeric coating. Although this type of system has met with large success, it nevertheless suffers from certain disadvantages. For example, heavy coatings are difficult to make because of the requirement to release carbon dioxide formed by the reaction of the isocyanate with water. Furthermore, since the chain extender is water, polymer characteristics are not as good as when polyols or polyamines are used as the chain extenders in the two part urethane coating compositions. It is therefore desirable to provide polymer forming compositions which are only one-component in nature, which are stable on storage and which can be cured in the absence of any excessive heating and without the specific addition of other materials.
It is disclosed in U.S. Pat. Nos. 3,743,626; 4,024,117; and 4,032,686 that certain oxazolidine compositions and polyfunctional aliphatic or aromatic isocyanates can be cured in the presence of moisture to give polymeric materials which are suitable for coatings. It is further disclosed that some such compositions can be stored in the absence of moisture without significant reaction between the oxazolidine and the isocyanate. Unfortunately, such compositions, as disclosed in the foregoing U.S. patents, have serious disadvantages. In particular, such coatings in general have very poor color. In addition, abrasion resistance is not as high as desired. Furthermore, the oxazolidine compounds used in these compositions were difficult to prepare due to long, complex or difficult synthesis routes. High solids in the coating compositions and good strength in the coatings obtained by good crosslinking were also difficult to obtain. Additional disadvantages of many of such compounds were that they were solids or waxes which were difficult to dissolve or disperse, they were unstable during storage or required polymer chains of specific and limited chemical structure, e.g., only those formed from certain lactones.
Additionally, except for certain compositions described in U.S. Pat. No. 4,381,388, such coatings are usually not high performance coatings, i.e., they do not have the hardness, strength, or weather resistance suitable for high performance applications such as those encountered in exterior environments.
In a more recent disclosure, e.g., U.S. Pat. No. 4,471,102, it was reported that a storage-stable one-component air curable urethane coating composition having improved color and abrasion resistance as well as better weatherability could be made by reacting an isophorone diisocyanate (IPDI) prepolymer with a deficiency of 2-hydroxyethyl oxazolidine (HEOX) to form a modified isocyanate containing prepolymer. The latter on application to a substrate in the presence of low levels of atmospheric moisture reacts to form an amino alcohol which co-reacts with the residual isocyanate contained in the modified prepolymer. The composition of this U.S. patent is, however, unfortunately limited to those products containing isophorone diisocyanate, mainly because of the stability of the materials on prolonged storage. In addition, weatherability was still not as good as desired.